Field of the Invention
The present invention relates to a gel lubricant, a rolling bearing filled with the gel lubricant, and a pivot assembly bearing provided with the rolling bearing. The present invention further relates to a hard disk drive including the pivot assembly bearing.
Description of the Related Art
A hard disk drive which is an information recording apparatus of a computer records information on a magnetic disk and retrieves the recorded information from the magnetic disk by moving a magnetic head over the magnetic disk. The magnetic head is generally attached to the tip of a swing arm which is swingably supported by a rolling bearing. The rolling bearing device which swingably supports the swing arm is called as a pivot assembly bearing.
Many kinds of lubricants have been suggested to facilitate the drive of the pivot assembly bearing. Examples of such lubricants include grease lubricants containing a base oil and a thickener composed of a urea compound or lithium soap. Japanese Patent Application Laid-open No. 2004-339447 discloses a grease lubricant containing a base oil, a thickener composed of a urea compound, and styrene block copolymer.
Although not used for the rolling bearing, Japanese Patent Application Laid-open No. 2002-327188 discloses a lubricant which contains a base oil and styrene block copolymer but does not contain any thickener. The lubricant disclosed in Japanese Patent Application Laid-open No. 2002-327188 is in a gel state and is an oil non-separating lubricant which completely prevents the separation of oil for a long period of time. Such a lubricant is suitably used for a sliding part of the gear of an electrical and electronic component and sliding parts of a switch and a relay connector.
In recent years, the magnetic head is required to move shorter distance in shorter time as the miniaturization, the growth in data density and the high speed are achieved for the hard disk drive. As a result, the magnetic head now repeatedly moves and stops very rapidly. Under such a situation that the pivot assembly bearing is overloaded, there is a demand for a lubricant capable of stably driving the bearing for a long period of time at a low torque while suppressing torque variation. Further, as the miniaturization is achieved for the hard disk drive and the pivot assembly bearing, the amount of the lubricant to be enclosed is decreased. Therefore, the need for increasing the ratio of the base oil which mainly contributes to lubrication is also increased to obtain a sufficient lubricating property even in a case that an amount of enclosed lubricant is small.
However, with respect to the grease lubricant disclosed in Japanese Patent Application Laid-open No. 2004-339447 containing the urea compound as the thickener, there is a possibility that the urea compound might be aggregated and solidified to cause great torque variation and thus might cause an operation failure of the magnetic head. While a rolling bearing generally rotates in one direction, the pivot assembly bearing performs an oscillating motion repeatedly rotating in forward and reverse directions at a very small angle. Thus, the grease lubricant is pushed to both lateral sides of rolling elements such as balls in the rolling bearing to generate a mass (channeling phenomenon), and the torque variation is caused when the ball attempts to ride over the mass. In the worst case, the bearing is locked and cannot be driven. In a case of the grease lubricant using the lithium soap as the thickener, the grease is always in a mixed state in the bearing once the pivot assembly bearing is driven (churning phenomenon). Thus, the torque is high right after the pivot assembly bearing is driven, and also the torque variation is great. Moreover, the grease lubricant is generally required to contain the thickener in an amount of around 12% by mass. Thus, the blending amount of the base oil is in a range of 85% by mass to 90% by mass and it is difficult to further increase the ratio of the base oil contributing to the lubrication.
Japanese Patent Application Laid-open No. 2002-327188 discloses a gel lubricant containing no thickener. The disclosed gel lubricant, however, is the oil non-separating lubricant which does not cause any separation of the oil. In the rolling bearing, the surface of a rotating ball is required to be covered with the oil of the lubricant. Thus, the lubricant disclosed in Japanese Patent Application Laid-open No. 2002-327188 with no oil separation cannot be applied as a lubricant for the rolling bearing.